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Jeff Ehme Spelman College jehme@spelman.edu Chris Christensen Northern Kentucky University christensen@nku.edu

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SkytaleCaesar cipher

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Vigenere Cipher 16 th century Playfair Cipher 19 th century Transposition Cipher

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KL47 KW-7

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ATMIFF

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Data Encryption Standard 1977 “New Directions in Cryptography” 1976

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Symmetric key block ciphers Public key ciphers Stream ciphers Post-quantum ciphers

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SubstitutionTransposition

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DROSX DOBOC DSXQW KDROW KDSMK VKCZO MDCYP ZELVS FUOIM SZROB CRKFO CZKBU ONSXD OBOCD LIWKD ROWKD SMCPK MEVDI SXDRO COMSZ ROBCK CKZZV SMKDS YXCYP WKDRO WKDSM CDRKD MKXLO ZBOCO XDONS XEXNO BQBKN EKDOM YEBCO CYPDO XSQXY BONRY GOFOB KBODR OWYNO BXZBS FKDOU OIMSZ ROBCD ROGYB URYBC OCYPM BIZDY QBKZR IWYNO BXZBS FKDOS OIMSZ ROBCK BOOAE KVVIW KDROW KDSMK VVISX DOBOC DSXQ

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Lester Hill (1891 – 1961)

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Symmetric Key vs Asymmetric key

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Symmetric Key Asymmetric Key

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If the solution to a problem can be quickly verified by a computer, can the computer also solve that problem quickly?

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Horst Feistel (1915 – 1990) Simplified DES: Wade Trapp and Lawrence Washington

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DES 64-bit block 56-bit key (8 parity bits) 8 S-boxes 6 x 4 16 rounds Feistel cipher 12-bit block 9-bit key 2 S-boxes 4 x 3 4 rounds Feistel cipher Simplified DES

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9 bits of key 111 010 110 Use the 8 bits on the left for k1 1110 1011 Rotate to the left one bit 110 101 101 Use the 8 bits on the left for k12 1101 0110 Rotate to the left 1 bit 101 011 011 Use the 8 bits on the left for k3 1010 1101 Rotate to the left 1 bit 010 110 111 Use the 8 bits on the left for k4 0101 1011

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Encryption Decryption

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EncryptionDecryption

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Backdoor?

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Perfect Security Random key, as long as the message, and used only once. Computational Security Today 112

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Skipjack declassified 1998 Clipper Chip 1993 -1996 64-bit block 80-bit key 32 rounds Unbalanced Feistel cipher

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Claude Shannon (1916 – 2001)

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“A Mathematical Theory of Communication” 1948 “The Theory of Secrecy Systems” 1949

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Diffusion: “… the statistical structure of [the message] which leads to its redundancy is dissipated into long range statistics.” Wire crossing, S-box Confusion: “… make the relation between the simple statistics of [the ciphertext] and the simple description of [the key] a very complex and involved one.” Permutation, P-box

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Many

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64-bit block 80- or 128-bit key 1 S-box 4 x 4 31 Rounds Substitution-Permutation Network

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How do they know it’s secure?

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Joan Daemen and Vincent Rijmen Simplified AES: Mohammad Musa, Edward Schaefer, and Stephen Wedig

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AES 128-bit block 128-bit, 192-bit, or 256-bit key 10, 12, or 14 rounds SPN 16-bit block 16-bit key 2 rounds SPN Simplified AES

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Serpent Twofish RC6 MARS

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Simplified IDEA Hoffman, N. 2007. “A Simplified IDEA Algorithm,” Cryptologia 31(2), 143 – 151. TEA Holden, J. 2013. “Demitasse: A ‘Small’ Version of the Tiny Encryption Algorithm and Its Use in a classroom Setting.” Cryptologia 37(1), 74 – 83.

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Stream ciphers

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Simplified DES Trappe, W. and Washington, L. Introduction to Cryptography with Coding Theory Simplified IDEA Hoffman, N. 2007. “A Simplified IDEA Algorithm,” Cryptologia 31(2), 143 – 151.

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PRESENT Boganov, A. and others. 2007. “PRESENT: An Ultra-Lightweight Block Cipher.” (Search the net.) Simplified AES Musa, A., Schaefer, E., and Wedig, S. 2010. “A Simplified AES Algorithm and Its Linear and Differential Cryptanalysis,” Cryptologia 27(12), 148 – 177.

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TEA Holden, J. 2013. “Demitasse: A ‘Small’ Version of the Tiny Encryption Algorithm and Its Use in a classroom Setting.” Cryptologia 37(1), 74 – 83. Cryptology course notes and student-produced software http://www.nku.edu/~christensen/1301cscmat483.htm

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